[diesel_john]

i have had a tough time isolating how much difference one change makes in the mileage. every part is so dependent on whats ahead of it and whats behind it in the air stream. i now have the truncated back, full belly pan except under the engine oil pan, single side skirts, and front splitter. gains are around +5 mpg. but other versions with full egg shaped front with air dam, full front and rear fendercovers ending at the rear bumper, double side skirts and no fastback gained as much as +9mpg.

IMHO the most important traits in the back are that all surfaces (top, bottom,sides) end at the same time with the same amount of pressure against them. And as the air leaves the trailng edge that it flows off exactly parallel to the directions of the vehicle, not curving inward towards a low pressure area. imho tapering the trailing edge only works best when the rear comes to a point. if the yarn tufts blow straight back you got it.